Scroll-type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 (Creux) discloses the basic construction of a scroll-type fluid displacement apparatus which includes two scroll members, each having a circular end plate and a spiroidal or involute spiral element. These scroll members are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, therefore, the fluid pockets change in volume. Since the volume of sealed off fluid pockets increases or decreases depending on the direction of the orbiting motion, the scroll-type fluid displacement apparatus is applicable to compress, expand or pump fluids.
In comparison with a conventional compressors of the piston type, a scroll-type compressor has certain advantages, such as fewer parts and continuous compression of fluid. However, one of the problems encountered in prior art scroll-type compressors has been ineffective sealing of the fluid pockets. Axial and radial sealing of the fluid pockets must be maintained in a scroll-type compressor in order to achieve efficient operation. The fluid pockets in the scroll-type compressor are defined by line contacts between the interfitting spiral elements and axial contacts between the axial end surface of the spiral elements and the inner surface of the end plates.
One prior art solution to the radial sealing problem is described in copending application Ser. No. 244,961, filed on Mar. 18, 1981. This application discloses an orbiting scroll rotatably supported on a crank pin through a bushing wherein the axis of the crank pin is radially offset or eccentric to the axis of the drive shaft. During operation of the apparatus, radial sealing is effected by the orbiting scroll being pushed against the fixed scroll due to the moment created by the differential between the driving point and reaction force acting point.
Furthermore, various techniques have been used in the prior art to resolve the sealing problem, particularly the axial sealing problem. For example, U.S. Pat. No. 3,994,635 (McCullough) discloses a scroll-type fluid displacement apparatus wherein the end surface of each spiral element facing the end plate of the other scroll member includes a groove formed along the spiral. A seal element is loosely fitted within the groove and an axially force urging device, such as a spring, is placed behind the seal element to urge the seal element toward the facing end surface of the end plate to thereby effect axial sealing.
In this construction of axial sealing mechanism, the contacting surface between inner end surface of end plate and the axial end surface of spiral element, i.e., end surface of seal element, is lubricated by lubricating oil contained in the gas which is taken into the fluid pockets. The lubricating oil flows along the groove with the gas because of the pressure difference between the areas adjacent outer end of the spiral element and the center of the spiral element.
However, this solution would not work satisfactorily in a scroll-type fluid displacement apparatus such as is shown in U.S. Pat. No. 4,303,379, wherein the radius of orbiting end plate is formed smaller than the radius of fixed end plate to reduce the diameter of compressor casing while keeping the same displacement capacity. In this apparatus the outer terminal portion of fixed spiral element can move out of contact with opposed orbiting end plate. The seal element in the groove of fixed spiral element therefore cannot extend along the entire length of the spiral element because the outer portion of seal element may interfere with the edge of orbiting end plate. Thus, the contact portion between inner end surface of the orbiting end plate and the outer terminal end portion of the fixed spiral element, in which a seal element is not disposed, is not lubricated by oil or gas. Interference between the end plate and outer terminal portion of spiral element may occur due to insufficient lubricating oil, thereby causing abnormal wear.